Epoxy resins can be formulated with a number of reagents to modify and enhance their performance. An important focus is on increasing the toughness of cured epoxies. There have been numerous studies on the use of engineering thermoplastics in epoxies for enhancing performance. The major advantage of thermoplastic-modified epoxy resins over elastomeric-modified systems is increased toughness along with no significant decrease in modulus or glass transition temperature (Tg). Performance of the final material is generally determined by the morphology of the material. Two-phase morphologies can be tougher than single-phase morphologies.
In general, the method for making epoxy resins containing thermoplastics involves a “solution process” where the thermoplastic is dissolved in hot epoxy resin to form a homogeneous solution. Phase-separated morphologies can be obtained by temperature induced phase separation and reaction induced phase separation during curing. In temperature-induced phase separation, the onset of phase separation occurs as the temperature is decreased. In reaction-induced phase separation, a second phase forms to produce multiphase morphology as the epoxy resins is curing.
The complexity of the solution process is illustrated in the temperature-composition phase diagrams for PPE (0.40 IV dl/g) epoxy resin (diglycidyl ether of bisphenol A; Epon 828) at 175° C. forming a homogeneous solution as reported by Venderbosch et al. [R. W. Venderbosch, H. E. H. Meijer, P. J. Lemstra, Polymer, 35, 4349 1994]. There is the onset of phase separation as indicated by the cloud point curve upon cooling of the solution. Below the cloud point curve, the solution undergoes liquid-liquid phase separation. However, on further cooling the phase-separated solution develops a Tg of approximately 100° C. Another phenomena in the two-phase region, is phase inversion as the PPE content is increased. At low PPE content, epoxy is the predominant material in the continuous phase with predominately PPE in the dispersed phase. In the range around 20 weight percent (wt %) PPE, co-continuous morphology occurs. With greater than 20 wt % PPE, the PPE is the predominant material in the continuous phase and epoxy is the dispersed phase. Terms such as “predominant material” or “PPE-rich” or “epoxy-rich” are used to imply that there is not always complete separation or segregation between the phases.
The temperature-composition phase diagram for a PPE epoxy resin is shown in FIG. 1. As depicted in FIG. 1, a variety of different composition/morphological zones are present. A description of these zones appears in Table 1. The location of the composition and cure temperatures needs to be considered with respect to these various zones. In fully cured epoxies, the thermoplastic can be the dispersed phase, the continuous phase, or can exhibit some level of co-continuous morphology depending on a number of factors.
TABLE 1Morphology Zones in phase diagramPPEEpoxyZone 1Single phasehomogeneous liquidZone 2Two phasesdispersed liquid phasecontinuous liquid phaseZone 3Two phasesdispersed solid phasecontinuous liquid phaseZone 4Two phasesco-continuousZone 5Two phasescontinuous liquiddispersed liquid phasephaseZone 6Two phasescontinuous solid phasedispersed liquid phase
Because of this morphological complexity, the use of thermoplastics in making epoxy resins can be complicated. As a result, there is a continuing need for materials and processes that can be used to improve epoxy resin toughness.